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The case of the man whose cancer came from a tapeworm

A Colombian man’s bizarre tumours weren’t human; their cells came from a tapeworm, a new study says.

An image of the Hymenolepis microstoma tapeworm, which looks identical to its close relative Hymenolepis nana, the dwarf tapeworm, which infects an estimated 75 million people worldwide. (XL30users / Peter Olson)

At first, Dr. Carlos Agudelo saw nothing strange in his new patient, an HIV-positive man who turned up at his clinic in January 2013 with a cough.

But when Agudelo found large masses in the man’s lungs and lymph nodes, he ordered biopsies. The results were unlike anything he had seen.

“The patient had a cancer,” said Agudelo, the head of infectious diseases at the Clinica Universitaria Bolivariana in Medellin, Colombia. “But the cancer was not produced by his cells.”

The cancer cells, in other words, were not human — they were from another species. The bizarre revelation triggered a three-year investigation, involving 18 researchers from around the world and ultimately leading back to a tiny parasite living inside the patient’s gut: a tapeworm.

The discovery marks the first time researchers have identified a cancer-like tumour inside a human that originally arose from a dwarf tapeworm, or Hymenolepis nana. On Wednesday, a case report of the 41-year-old Colombian patient was published in The New England Journal of Medicine.

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H. nana is the most common tapeworm in the world, carried by an estimated 75 million people. Many infected never show symptoms but people with compromised immune systems, like HIV patients, can develop complications.

Cancerous tumours, however, were not among the complications doctors knew to look for. So when Agudelo saw his patient’s bizarre biopsy results, he was stumped. He reached out to international experts for help, including the U.S. Centers for Disease Control and Prevention.

“It really was a unique case that we had not seen before,” said lead author Dr. Atis Muehlenbachs, a CDC pathologist who investigates unexplained diseases. “These were cells that were behaving like a cancer, growing, invading tissues ... but they were too small to be human cells. It just didn’t make sense.”

The researchers suspected an infection. Muehlenbachs’ initial hunch was to look for an amoeba-type organism or “plasmodial slime mould.” But after five months and dozens of test, a DNA sequencing test unexpectedly showed a match — for the dwarf tapeworm.

Muehlenbachs didn’t believe the result at first but further tests confirmed the diagnosis. He also found earlierstudies reporting similar cases, including an HIV-positive man in San Francisco who developed cancer-like growths in 1994 that were later linked to the tapeworm.

That earlier case was not definitively shown to be cancer and appeared more like an “overgrowth” of tapeworm tissue, Muehlenbachs said.

Dwarf tapeworms are unusual in that they have the unique ability to complete their life cycle inside a single animal host, according to Peter Olson, a tapeworm expert and parasitologist with the Natural History Museum in London, England. Olson, who previously studied the San Francisco case, joined the international investigation into the Colombian patient after being contacted by the CDC.

Other terrestrial tapeworms require two animal hosts: one for the larval stage (perhaps a beetle) and another for their metamorphosis into an adult worm (perhaps a mouse which eats the beetle and ingests the larvae).

The dwarf tapeworm can spend its entire life cycle inside the human gut. Olson hypothesized that in people with weakened immune systems, however, the tiny larvae manages to burrow through the gut wall, somehow winding up inside the lymphatic system.

In this new, strange environment, the larvae can’t develop into its adult form but its cells might grow exponentially and “effectively form little tumours,” Olson said.

“This is the case where the tapeworms are the cancer.”

For the Colombian patient, the researchers’ discovery came too late. When the CDC shared their findings with Agudelo, the patient’s kidneys were failing and he died three days later.

The researchers hope their paper will alert doctors around the world to this rare but deadly consequence of tapeworm infections — especially in poor countries where both HIV and tapeworm infections are common.

The case is also a reminder that many unanswered questions remain when it comes to microbes and their capabilities, said Dr. David Relman, a microbiologist and infectious disease expert with Stanford University.

Relman, one of the investigators of the San Francisco case, said some parasites take advantage of hosts with weakened immune systems. The immunocompromised, therefore, are often “canaries in the coal mine” when it comes to discovering surprising new diseases.

“What other kinds of stories are there out there, waiting to be discovered?” he asked.

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